The proposed studies attack unresolved questions about the fundamental organization of the cochlear spiral ganglion and its input to the brainstem. Using the cat as the animal model, certain inadequately substantiated concepts of the neuronal wiring of the mature cochlea will be re-examined. A combination of auditory evoked response, physiological microelectrode recording and modern tracer techniques along with extensive light and electron microscopic analyses will be employed. Special attention will be given to the morphology, distribution and connections of the small unmyelinated "Type II" ganglion cells as contrasted with the more common "Type I" cells. The nature of neurons which resist retrograde degeneration following transection of the cochlear nerve, or long term total or partial destruction of the organ of Corti by neomycin sulfate will be re-evaluated. The suggestion in recent literature that only the inner hair cells (via the Type I ganglion cells) have neuronal input to the central auditory nervous system, while the outer hair cell system is not connected to the cochlear nuclei, will be tested directly with modern neuroanatomical tracer methods. In other experiments, the central distribution of afferents from the two ganglion cell populations will be delineated. Further development and documentation of a reproducible animal model which has large sectors of the organ of Corti with inner or outer hair cells selectively preserved following ototoxic drug treatment will be provided.